1. Field of the Invention
The present invention relates to a method for releasing a micromechanical structure, and more particularly, to a method of etching an amorphous silicon layer during the microelectromechanical structure (MEMS) process.
2. Description of the Related Art
The use of selective etchants to remove sacrificial layers or regions in a multilayer structure without removal of an adjacent layer or region is a common step in the manufacture of semiconductor devices and MEMS. MEMS have found applications in inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio frequency communications, and the range of applications for these structures continues to grow. One example of such a structure is a reflective spatial light modulator, which is a device consisting of a planar array of electrostatically deflectable mirrors, each microscopic in size. The device is used as a micro-display system for high resolution and large screen projection. The sacrificial layer in such a device is the layer over which the mirror material is deposited. Once the mirror structure is formed, the sacrificial layer is removed to leave gaps below the mirrors and microhinge along one edge of each mirror to join the mirror to the remainder of the structure. The gap and the microhinge provide the mirror with the freedom of movement needed for its deflection. Devices of this type are described in, for example, U.S. Pat. No. 6,356,378.
The success of a manufacturing procedure for structures involving sacrificial layers depends on the selectivity of the etching process. The thickness and lateral dimensions of the layers, and in the case of the deflectable mirror structures, the width of the gap and the integrity of the microhinges, are all critical to achieve uniform microstructure properties and a high yield of defect-free product. One of the critical factors is the quality of the etching process. Performance, uniformity and yield can all be improved with increases in the etch selectivity of the sacrificial layer relative to the adjacent functional layers.
Among the etchants that are used for the removal of sacrificial layers or regions in etching procedures are noble gas fluorides and halogen fluorides. These materials, used in the gas phase, selectively etch silicon relative to other materials such as silicon-containing compounds, metallic elements, and compounds of metallic elements. The selectivity is not infinite, however, and would vary widely with the specific equipment, process, materials and reaction conditions. Xenon difluoride (XeF2), for example, has demonstrated selectivity as high as 400:1 to 500:1. Nevertheless, the XeF2 etcher is a special and expensive facility for current semiconductor processes, thereby increasing additional equipment and material costs. In addition, when the sacrificial layer is very thick (e.g. above 2000 Å), XeF2 etching requires longer etching time, thereby reducing manufacturing efficiency.
In U.S. Patent Application No. 20020197761, Patel et al disclose a method for releasing a micromechanical structure. The method performs a first etch to remove a portion of the sacrificial material, wherein the first etch provides an etchant of CH4 or C4H8 so as to allow the etchant to physically, or chemically and physically, remove the portion of the sacrificial material. Then, a second etch is performed to remove additional sacrificial material, wherein the second etch provides an etchant of XeF2, IF5, IF7 or BrF3 that chemically but not physically etches the additional sacrificial material. Though effective, this method nevertheless requires special and expensive dry etchers, such as the XeF2 etcher.
In U.S. Pat. No. 6,290,864, Patel et al disclose a gas-phase etching procedures for the manufacture of MEMS. The method, utilizing etchant gases that are noble gas fluorides or halogen fluorides, is performed with greater selectivity toward the silicon portion relative to other portions of the microstructure by the addition of non-etchant gaseous additives to the etchant gas. Thus, this method still requires special and expensive dry etchers, such as the XeF2 etcher.
In U.S. Pat. No. 6,396,619, Huibers et al disclose a deflectable spatial light modulator (SLM). The patent does not teach how to etch the sacrificial layer of amorphous silicon.